1. Field of the Invention
This invention relates generally to heating systems and, more particularly, to a heating system which includes a solid fuel burner (i.e. a wood or coal burning stove) used in conjunction with a fluid (gas or oil) fired hot water furnace.
2. Description of the Prior Art
At least two methods of integrating a wood fired heater into an existing gas or oil heating system are known; i.e. a series system and a parallel system. The control systems for both arrangements are substantially identical varying only with respect to water (aquastat) settings for the two burners and, in some cases, modifications of these settings based on external temperature conditions. The intended result is the achievement of a higher water temperature in the wood fired heater than the high limit point of the aquastat controlling the gas or oil burner to minimize the time during which the temperature of the water in the gas or oil fired furnace is equal to or below the temperature at which its burner is activated. This requires a poor compromise in the setting of the aquastat controls on the gas/oil burner.
A typical control system for the gas/oil fired system has a twin level aquastat, one of which has a settable make break differential and is referred to as the low-temp setting. The control setup can vary depending upon whether the system utilizes a tankless or external heat exchanger for domestic hot water. However, the majority of installed systems are tankless meaning that the effective domestic hot water reservoir is limited to the heat exchanger inserted in the water jacket of the water heater.
In the tankless system, the low-temp aquastat is coupled so as to activate the burner independently of zone demand and, thus, maintain water temperature between the limits established by the differential adjustment which, in turn, maintains the low limit for domestic hot water. With the activation of a heating zone, the low-temp aquastat is bypassed and the burner fires until the hi-temp setting is reached. It is relatively standard procedure to control the circulator pump from the make-on-low limit contact of the low-temp aquastat to avoid circulating water at low temperature which is both inefficient and also impacts domestic hot water temperature. This feature has a negative impact on the management of over-temperature conditions in the wood fire heater.
For example, caloric input to a solid fuel heater does not immediately cease upon draft closure. Therefore, the water temperature may rise to above boiling and internal boiler pressure may activate a relief valve. To avoid this, the wood heater controls should include a separate aquastat control for detecting a temperature rise which approaches the boiling point (which varies according to water system pressure) and consequential activation of one of the zone controls which serves to dump the excess heat. However, if the gas/oil heater control is arranged to prevent circulator operation below the low-temp setting, it would be necessary for the gas/oil burner to raise the temperature of the gas/oil boiler prior to any dumping of excess heat from the wood boiler. This creates the possibility, in a series arrangement, of completely disabling the entire heating system upon the loss of the gas/oil fuel supply since, once the low-temp aquastat opens (closing the circulator circuit and calling for burner activation), the only source of heat would be from the wood burner. However, without the circulation, no heat would be transferred between boilers. In a parallel arrangement, the temperature of the gas/oil fired boiler is maintained by convection at a temperature approaching that of the wood fired boiler other than for peak demands on domestic hot water.
A problem in both the series and parallel tankless systems is that domestic hot water temperature cannot be maintained by the wood boiler in the absence of zone demand because (1) in the series arrangement, there is virtually no transfer of heat from the wood boiler to the gas/oil boiler except with the circulator pump running; and (2) in the parallel arrangement, heat transfer by connection is not equivalent to the BTU demand of domestic hot water.